Electrical circuits requiring high power handling capability while operating at high frequencies such as radio frequencies, S-band and X-band have in recent years become more prevalent. Because of the increase in high power, high frequency circuits there has been a corresponding increase in demand for transistors that are capable of reliably operating at radio frequencies and above while still being capable of handling higher power loads. Previously, bipolar transistors and power metal-oxide semiconductor field effect transistors (MOSFETs) have been used for high power applications but the power handling capability of such devices may be limited at higher operating frequencies. Junction field-effect transistors (JFETs) were commonly used for high frequency applications but the power handling capability of previously known JFETs may also be limited.
Metal-semiconductor field effect transistors (MESFETs) have been developed for high frequency applications. The MESFET construction may be preferable for high frequency applications because only majority carriers carry current. The MESFET design may be preferred over MOSFET designs because the reduced gate capacitance permits faster switching times of the gate input. Therefore, although all field-effect transistors utilize only majority carriers to carry current, the Schottky gate structure of the MESFET may make the MESFET more desirable for high frequency applications.
A conventional MESFET structure is illustrated in FIG. 1. The MESFET includes a first epitaxial layer 12 of p-type conductivity on a single crystal bulk silicon carbide substrate 10 of either p-type or n-type conductivity. A second epitaxial layer 14 of n-type conductivity is grown on the first epitaxial layer 12. Wells 16 and 18, of n+ conductivity are formed in the second epitaxial layer 14. A conducting plane 32 may be formed on the opposite side of the substrate from the first epitaxial layer 12.
Ohmic contacts 20 and 22, are formed on wells 16 and 18 to create a source contact 20 and a drain contact 22. A Schottky gate contact 24 is formed on the second epitaxial layer 14 between the source contact 20 and the drain contact 22, and metal overlayers 26, 28 and 30 are formed on the source and drain contacts 20 and 22 and the Schottky gate contact 24. The conductivity of the channel region of the second epitaxial layer 14 between the source well 18 and the drain well 16 is controlled by a bias voltage applied to the gate contact 24. For example, when a sufficiently large negative voltage is applied to the gate contact 24, a depletion region beneath the gate contact extends to the p-type first epitaxial layer 12, pinching off the channel in the second epitaxial layer 14.
In addition to the type of structure, and perhaps more fundamentally, the characteristics of the semiconductor material from which a transistor is formed also affects the operating parameters. Of the characteristics that affect a transistor's operating parameters, the electron mobility, saturated electron drift velocity, electric breakdown field and thermal conductivity may have the greatest effect on a transistor's high frequency and high power characteristics.
Electron mobility is the measurement of how rapidly an electron is accelerated to its saturated velocity in the presence of an electric field. Semiconductor materials which have a high electron mobility are typically preferred because more current can be developed with a lower field, resulting in faster response times when a field is applied. Saturated electron drift velocity is the maximum velocity that an electron can obtain in the semiconductor material. Materials with higher saturated electron drift velocities may be preferred for high frequency applications because the higher velocity translates to shorter times from source to drain.
Electric breakdown field is the field strength at which breakdown of the Schottky junction and the current through the gate of the device suddenly increases. A high electric breakdown field material may be preferred for high power, high frequency transistors because larger electric fields generally can be supported by a given dimension of material. Larger electric fields allow for faster transients as the electrons can be accelerated more quickly by larger electric fields than by smaller fields.
Thermal conductivity is the ability of the semiconductor material to dissipate heat. In typical operations, all transistors generate heat. In turn, high power and high frequency transistors usually generate larger amounts of heat than small signal transistors. As the temperature of the semiconductor material increases, the junction leakage currents generally increase and the current through the field effect transistor generally decreases due to a decrease in carrier mobility with an increase in temperature. Therefore, if the heat is dissipated from the semiconductor, the material will remain at a lower temperature and be capable of carrying larger currents with lower leakage currents.
High frequency MESFETs may be manufactured of n-type III-V compounds, such as gallium arsenide (GaAs) because of their high electron mobilities. Although these devices provide increased operating frequencies and moderately increased power handling capability, the relatively low breakdown voltage and the lower thermal conductivity of these materials have limited their usefulness in high power applications.
Silicon carbide (SiC) has been known for many years to have excellent physical and electronic properties which should theoretically allow production of electronic devices that can operate at higher temperatures, higher power and higher frequency than devices produced from silicon (Si) or GaAs. The high electric breakdown field of about 4×106 V/cm, high saturated electron drift velocity of about 2.0×107 cm/sec and high thermal conductivity of about 4.9 W/cm-° K indicate that SiC would be suitable for high frequency, high power applications.
MESFETs having channel layers of silicon carbide have been produced on silicon substrates (See, e.g., U.S. Pat. Nos. 4,762,806 to Suzuki et al. and 4,757,028 to Kondoh et al.). Because the semiconductor layers of a MESFET are epitaxial, the layer upon which each epitaxial layer is grown affects the characteristics of the device. Thus, a SiC epitaxial layer grown on a Si substrate generally has different electrical and thermal characteristics then a SiC epitaxial layer grown on a different substrate. Although the SiC on Si substrate devices described in U.S. Pat. Nos. 4,762,806 and 4,757,028 may have exhibited improved thermal characteristics, the use of a Si substrate generally limits the ability of such devices to dissipate heat. Furthermore, the growth of SiC on Si generally results in defects in the epitaxial layers that result in high leakage current when the device is in operation.
In general, MESFETs on SiC substrates have exhibited improved thermal characteristics over previous devices because of the improved crystal quality of the epitaxial layers grown on SiC substrates. However, to obtain high power and high frequency it may be necessary to overcome the limitations of SiC's lower electron mobility.
Similarly, commonly assigned U.S. Pat. No. 5,270,554 to Palmour describes a SiC MESFET having source and drain contacts formed on n+ regions of SiC and an optional lightly doped epitaxial layer between the substrate and the n-type layer in which the channel is formed. U.S. Pat. No. 5,925,895 to Sriram et al. also describes a SiC MESFET and a structure that is described as overcoming “surface effects” which may reduce the performance of the MESFET for high frequency operation. Sriram et al. also describes SiC MESFETs that use n+ source and drain contact regions as well as a p-type buffer layer. SiC MESFETs are also discussed in U.S. Pat. No. 6,686,616 to Lipkin et al.
Furthermore, conventional SiC FET structures may provide constant characteristics during the entire operating range of the FET, i.e. from fully open channel to near pinch-off voltage, by using a very thin, highly doped channel (a delta doped channel) offset from the gate by a lightly doped region of similar conductivity type. Delta doped channels are discussed in detail in an article by Yokogawa et al. entitled Electronic Properties of Nitrogen Delta-Doped Silicon Carbide Layers, MRS Fall Symposium, 2000 and an article by Konstantinov et al. entitled Investigation of Lo-Hi-Lo and Delta Doped Silicon Carbide Structure, MRS Fall Symposium, 2000. However, further improvements may be made in SiC MESFETs.
For example, it may be important that SiC MESFETs have high breakdown voltages and relatively low leakage currents if they are used in high efficiency, high power, high linearity radio frequency (RF) applications. In an attempt to provide high breakdown voltages, devices have been provided having highly compensated substrates, such as vanadium doped semi-insulating SiC. These devices typically provide adequate breakdown voltages as well as low leakage currents, but may sacrifice device performance due to unwanted trapping effects in the substrate. Furthermore, devices having highly doped p-type layers under the channel of the FET have been provided and have been successful in providing good electron confinement and low leakage currents. However, these devices generally contain excessive parasitics that may degrade the RF performance of the device. Accordingly, further improvements may be made with respect to existing SiC FET devices such that they may provide improved breakdown voltages without sacrificing other performance characteristics of the device.